"When they moved forward to confirm the planning at the front lines in order to visualise what they had discussed over a map, they came under immediate and effective mortar and machine gun fire."

Rouleau said the Canadians used sniper fire in self-defence and successfully "neutralised" the jihadists' positions without taking any casualties. The exact location of the exchange was not released for operational security reasons.

Canada joined a US-led coalition against the jihadi group last year, sending six CF-18 fighter jets, a refuelling tanker aircraft, two surveillance planes and about 600 airmen and airwomen to carry out air strikes in Iraq.

It has deployed 69 Special Forces soldiers on a training and advisory mission to help local troops fight back against Isis.

Rouleau said that although about 20% of Special Forces activities are carried out at the front lines, they are not participating in active combat and the shooting incident fell within the scope of their mission.

"This is the first time this has happened since our arrival and our reaction is wholly consistent with the inherent right of self-defence," the senior officer told a media briefing.

Nevertheless opposition figures argued that the fire exchange came in stark contrast with the government line that Canadian troops were not to be involved in ground combat or accompany Iraqi forces into battle.

"If we're at the front lines and calling in air strikes and we're engaged in firefights because we're subject to machine-gun fire, that's not what Canadians were told," New Democrat lawmaker Jack Harris told The Globe and Mail.

The government maintained the incident didn't exceed the scope of the mission.

Jason MacDonald, a spokesman for Prime Minister Stephen Harper, told local media: "A combat role is one in which our troops advance and themselves seek to engage the enemy physically, aggressively, and directly. That is not the case with this mission.

"This mission is one in which they are providing advice and assistance to Iraqi forces only.... That said, we have always been clear that while this is a low-risk mission, it is not without risk and our forces on the ground will protect themselves if fired on in the course of carrying out their mission."